The present invention concerns an intrusion detector with a sensor having at least one infrared-sensitive sensor element and several infrared reflector segments arranged on at least one supporting structure which focus infrared radiation from a number of separate detection zones on to a common sensor.
Such detectors record the presence of objects or persons, such as an intruder or burglar in a monitored room or area by detecting the infrared radiation emitted by the object or person. Since a monitored area is divided into a number of detection zones separated by neutral zones, every movement by an intruder crossing the room produces a characteristic modulation of the infrared rays which is picked up by the sensor. By means of appropriate sensors, which can comprise several sensor elements connected in a specific manner such as dual sensors, the typical modulation of a person moving through the detection zones can by means of evaluating circuits indicate the presence of an intruder and activate an alarm signal. Such intruder detectors are not only required to detect and signal the presence of intruders in a monitored area with certainty while remaining immune to any attempt to sabotage the system, but also to avoid false alarms.
For the creation of the required separated detection zones U.S. Pat. No. 3,703,718 calls for reflector segments to be arranged next to each other on a common supporting structure in two rows one above the other. As only two corresponding rows of detection zones are provided, coverage of the room to be monitored with detection zones is inadequate, so that with skill, an intruder could cross a room without being detected and signalled.
For better coverage of the protected area with detection zones, CH 591 733 or DE 26 53 111 show that reflector segments must be so designed and arranged as to create a number of beam-shaped detection zones so that a larger protection area can be monitored with the same number of reflector segments on a common supporting structure. EP 50 751, DE 27 19 191 or U.S. Pat. No. 3,923,383 also show that a number of reflector segments on a common supporting structure can be arranged in the form of a multi-facetted mirror. Although here a monitored area can be covered relatively densely with the correspondingly large number of detection zones, such arrangements are not adapted to the given shape and dimensions of a room to be protected.
However, the above-mentioned reflector segment arrangement has the disadvantage that the focal lengths of all reflector segments are the same, so that a person further away produces a smaller image on the sensor than a person near the detector. This leads to variable detector sensitivity for persons within detection zones which cover areas at varying distances from the detector. With the usual arrangement of such detectors below the ceiling of the room, sensitivity depends on the angle of inclination of the detection zone from the horizontal plane, so that, e.g. in detection zones with a steep angle of inclination covering a room area close to the detector, detection sensitivity is reduced, which in practice is usually not wanted.
EP 191 155 or U.S. Pat. No. 4,339,748 specify that adjacent reflector segments should be arranged in three rows one above the other. The focal lengths of the individual rows of reflector segments are thus varied and adapted to the respective detection distance. However, they are the same within the individual rows. For this purpose the rows of reflector segments must be arranged on several different supporting structures so that the entire reflector arrangement has a complicated shape. An arrangement of reflector segments in a few rows does not provide adequate room coverage so that such a detector is not completely sabotage-proof. As the focal length is the same within one row of reflector segments, precise modification of the detection zone pattern to the specific form and dimensions of a room or area to be monitored is normally not given.
The present invention endeavors to eliminate the acknowledged disadvantages of the prior art and especially to provide an intrusion detector as described at the outset which has improved detection sensitivity and detection reliability using a simplified design and which in particular provides better and more uniform coverage for a given room or area to be monitored with detection zones. So that the detector cannot be outwitted easily, the detection zone pattern is adapted to the shape and dimensions of the room or area to be protected and the detection sensitivity for one person in the individual detection zones is virtually independent of the detector's detection distance.